Synopsis The Piper Navajo Chieftain departed Mary's Harbour en route to Fox Harbour, Newfoundland. The initial brake application during the landing roll at Fox Harbour resulted in normal braking action. However, subsequent brake applications resulted in progressively greater brake pedal travel and less braking response. The captain took control and ground looped the aircraft rather than overrun the runway and descend a 75-foot rock embankment. The flight crew and the three passengers, all uninjured, evacuated the aircraft and walked to the airport terminal. The Board determined that brake fluid boiling was the most probable cause of the brake failure. Contributing to this occurrence was the high energy landing in Mary's Harbour, the insufficient brake cooling time between the Mary's Harbour and Fox Harbour landings, the flight crew's braking technique, and the standard brake system installed on the aircraft. 1.0 Factual Information 1.1 History of the Flight The aircraft, a Piper Navajo Chieftain, departed St. Anthony on a scheduled 28-minute flight to Mary's Harbour, Newfoundland, about 66 miles to the north. Nine minutes after landing in Mary's Harbour, the aircraft departed for a short flight to Fox Harbour, eight miles east. The aircraft touched down on runway 05 at Fox Harbour, about 200 feet beyond the threshold, and the co-pilot, who was the pilot flying (PF)1, applied the brakes. Subsequent brake applications resulted in progressively greater brake pedal travel and less braking response. The captain, who was the pilot not flying (PNF), took control of the aircraft and ground looped it, rather than overrun the airstrip and descend a 75-foot rock embankment. The flight crew and three passengers evacuated the aircraft uninjured. See Glossary for all abbreviations and acronyms. Units are consistent with official manuals, documents, reports, and instructions used by or issued to the crew. All times are NDT (Coordinated Universal Time (UTC) minus 3 1/2 hours) unless otherwise stated. The accident occurred at latitude 5222'N and longitude 5541'W, at an elevation of 75 feet above sea level (asl)2, at approximately 0900 Newfoundland daylight time (NDT)3 during the hours of daylight. 1.2 Injuries to Persons 1.3 Damage to Aircraft The aircraft sustained substantial damage during the landing event. 1.4 Other Damage 1.5 Personnel Information 1.6 Aircraft Information Manufacturer - Piper Aircraft Corporation Type and Model - PA-31-350 Navajo Chieftain Year of Manufacture - 1974 Serial Number - 317405221 Certificate of Airworthiness (Flight Permit) - Valid Total Airframe Time - 9,368 hr Engine Type (number of) Lycoming TIO-540-J2BD - (2) Propeller/Rotor Type (nubmer of) Hartzell HC-E3YR-2ATF - (2) Maximum Allowable Take-off Weight - 7,250 lb Recommended Fuel Type(s) - 100/130 minimum Fuel Type Used - 100 LL The aircraft was certified, equipped and maintained in accordance with existing regulations and approved procedures. The aircraft was originally type certified for a maximum take-off and landing weight of 7,000 pounds. However, an approved modification to the aircraft, a vortex generator kit, increased the maximum take-off weight to 7,250 pounds. The modification did not change the maximum landing weight of 7,000 pounds. The aircraft's maximum allowable landing weight was exceeded by 125 pounds during the Mary's Harbour landing. The aircraft weight and centre of gravity during the landing at Fox Harbour were within the prescribed limits. 1.7 Meteorological Information The Mary's Harbour automated weather observation system (AWOS) reported weather at the time of the occurrence was 1,300 feet scattered, visibility five miles in rain showers, and light winds. The Fox Harbour weather, reported by the flight crew, was visibility three miles in rain showers, fog patches, and light and variable winds. 1.8 Aerodrome Information The Fox Harbour airport, which is operated by the Government of Newfoundland, has a 2,200- by-75 foot gravel runway. A 100-foot soft gravel surface extends beyond the departure end of runway 05, followed by a rock embankment that descends 75 feet. 1.9 Flight Crew Actions 1.9.1 Control of the Aircraft The PF is responsible for controlling the aircraft. When there is a need to exchange control of the aircraft between pilots, the procedure that should be followed is for the pilot taking control to call I have control, and for the pilot relinquishing control to call you have control. This control exchange can be initiated by either pilot. Proper verbal transfer of control from the PF to the PNF did not take place during the Fox Harbour landing. However, the PNF, the captain, did take control of the aircraft and identified that he also had no braking action. The company procedure for an overshoot is for the pilot controlling the aircraft to advance the throttles. During the landing at Fox Harbour the captain called overshoot, but did not advance the throttles. The captain then decided he did not have sufficient runway remaining to carry out an overshoot, so he ground looped the aircraft before it went past the departure end of the runway and over the embankment. 1.9.2 Braking Technique The flight crew were using a braking technique of brake on-off-on (pumping the brakes) after aircraft touchdown. An alternative braking technique would have been to apply the brakes after touchdown and maintain positive brake pressure until the aircraft had come to a stop. The latter technique provides a shorter landing distance. Also, the brake system, due to a higher constant brake pressure, could absorb more heat before the brake fluid would boil. The flight crew believed that their braking technique resulted in less wear and longer life to the aircraft brake system components. This technique was an accepted practice used by some of the operator's pilots. The operator did not have a Standard Operating Procedure (SOP) manual for this aircraft. 1.10 Wreckage and Impact Information 1.10.1 General The aircraft was substantially damaged during the landing event. The ground loop overloaded the aircraft's left main landing gear downlock and caused it to fail and the gear to collapse. The left engine propeller was damaged when it struck the ground after the landing gear collapsed. The left wing panels were visibly wrinkled on the outboard six feet of wing and the left flap was damaged. The left stabilizer and elevator were also damaged, as was the fuselage structure below the main cabin door. 1.10.2 Brake Examination After the occurrence, the brake system was checked for damage and proper operation. There was no evidence of brake fluid leaks from the brake system components. When the brakes were applied, the pedal response was spongy, similar to the response that is felt when air is in the brake system. When the pedals were pumped and then held, the pedal pressure would remain hard. The aircraft's main wheel brake discs and the wheel brake assemblies, including the brake linings, brake cylinders, and brake torque plates, were removed from the aircraft and transported to the Regional Wreckage Examination Facility in Moncton, New Brunswick. The following observations were made: Brake discs: Both brake discs were coned beyond the manufacturer's .015-inch limit. The left wheel brake disc was coned .139 inches and the right wheel brake disc was coned .054 inches. The left disc was marginally thinner than the right and the left disc had accumulated more landing cycles since installation. Both brake disc thicknesses were within the limits specified by the brake manufacturer's Component Maintenance Manual (CMM). Wheel Brake Cylinders: Both brake cylinders were intact and showed no evidence of cracks or external leakage. Brake Linings/Left Brake: All six brake linings met and exceeded the minimum thickness requirements specified by the CMM. The steel backing on some of the linings was warped and bluish in colour. Pressure Plate/Left Brake: Three brake linings are attached to the pressure plate and are located on the brake piston side of the brake disc. The linings are attached to pins that are riveted to the pressure plate. The pressure plate was warped, all six pins were loose in the pressure plate, and several pins had elongated their respective holes. Brake Linings/Right Brake: All six brake linings had minimal material remaining. Two of the three back plate linings were worn to the point that the retaining pin hole outlines were visible on the lining face. The CMM states that the lining is worn beyond limits when the holes for the pins are visible on the brake lining surface. Figure 3 -Brake assembly on brake disc The three pressure plate linings were worn to near minimum thickness limits. Pressure Plate/Right Brake: The right brake pressure plate was warped but the brake lining retaining pins were secure in the plate. 1.11 Brake Design and Braking Response 1.11.1 Brake Design The aircraft's wheel and brake (40-102/30-68 series) normal land kinetic energy capacity is rated at 700,000 ft-lb. This indicates that it will complete 35 dynamometer stops at that energy and at a deceleration rate of 10 ft/sec/sec. One dynamometer stop is considered equivalent to the kinetic energy that is produced during one short field landing sequence. Therefore, new brake linings should last for a minimum of 35 landings before lining replacement is required. The 30-68A standard brake was originally installed on the Piper Navajo, which had a 6,500-pound maximum landing weight with a speed of about 70 knots in landing configuration. The landing weight increased to 7,000 pounds with a speed of 74 knots in landing configuration when the Navajo Chieftain was introduced. Although the same brake was approved for Navajo models, a heavy-duty brake was available for the Chieftain that was rated to a higher energy capacity. Although the 30-68 series aircraft brake is rated at 700,000 ft-lb, it has the capacity to operate above this brake rating. Nevertheless, when the energy produced is higher than the design rating and the brake has not cooled sufficiently to dissipate the stored energy, any further braking can produce brake fade, brake disc coning, and abnormal brake response. All brake cylinders incorporate insulators that provide protection against heat transfer from the brake disc to the brake cylinders. 1.11.2 Brake Lining Conditioning Brake lining conditioning is a manufacturer's recommended procedure that should be followed after new linings are installed on the aircraft. Two consecutive full-stop braking applications are performed at a speed of 30 to 35 knots to glaze the brake lining material and provide optimum lining service life. If the conditioning is not done, the lining service life will decrease and the brakes will be less effective. Also, there will be a corresponding hard pedal feel, and a greater- than-normal pedal effort will be required to decelerate the aircraft. 1.11.3 Brake Heating If the brake discs are subjected to excessive heat during a landing, the brake discs can cone. As the disc starts to cone, the running clearance between the linings and the disc decreases. Also, the misalignment between the disc and linings causes the pressure distribution to become non- uniform, producing less braking action. Excessive heat energy can be generated if the brake has not had sufficient time to cool between stops. This residual energy is stored in the disc and is added to the energy created during the next stop. The resulting high temperatures can exceed the brake insulation capabilities and cause the brake fluid behind the brake cylinder pistons to boil. Since a gas is compressible, any continued brake application will produce excessive pedal travel and poor or no brake response. A brake fluid will reach a higher temperature without boiling when the brake fluid is under pressure. If the fluid is close enough to its boiling point when under pressure, the fluid will boil when the pressure is removed. 1.12 Brake Kinetic Energy on Landing 1.12.1 Kinetic Energy Formula Every brake is designed to a particular brake capacity measured in ft-lb. The formula for calculating the kinetic energy (KE) that can be produced at the brake during a landing event is as follows: KE = LW x (LS x LS) x .0443 divided by X, where LW = aircraft landing weight in pounds, LS = aircraft landing speed in knots, and X = The number of brake assemblies per aircraft. 1.12.2 Kinetic Energy Calculations The following calculations use the aircraft's landing weights computed from the occurrence flight load sheets. Two Mary's Harbour landing speeds were used to highlight the importance of landing speed in determining the brake kinetic energy. Calculation No. 2 uses the highest landing speed that the crew feels might have been attained. Mary's Harbour Calculation No. 1 KE = LW x (LS x LS) x .0443 divided by X = 7125 x 75 x 75 x .0443 divided by 2 Mary's Harbour Calculation No. 2 KE = LW x (LS x LS) x .0443 divided by X = 7125 x 85 x 85 x .0443 divided by 2 Although the maximum approved landing weight was exceeded by 125 pounds, the overweight landing increased the energy developed by less than 2 per cent. KE = LW x (LS x LS) x .0443 divided by X = 6286 x 80 x 80 x .0443 divided by 2 1.13 Additional Information 1.13.1 Maintenance Information An Event No. 3 inspection and brake system repairs were carried out on the day preceding the accident. The repairs included replacing all O-rings in both wheel brake assemblies and the parking brake valve. The operator's Parts and Rectification Sheet first identified that 6 of the 12 brake linings were replaced and then the sheet was corrected to read 9. Aircraft Maintenance Record sheet No. 09916 reads that the pads (brake linings) were worn beyond limits, and the rectification reads that the pads were replaced. It did not indicate how many pads were replaced. The Aircraft Maintenance Engineer (AME) did not notice to what extent the left brake disc was coned when he replaced the left brake linings. However, he was able to tighten the four bolts and still rotate the tire. The captain helped the AME bleed the brake system by pumping the brakes from both cockpit positions. The brake lining conditioning procedure was not carried out on the occurrence aircraft after the linings were replaced, although the brakes were tested for proper operation. The flight crew did not experience any abnormal aircraft brake conditions prior to the Fox Harbour landing. The aircraft records for the previous 100 hours of aircraft operation indicated that brake lining service life ranged from 80 to 160 landings before the linings were replaced. The aircraft had completed two landings since the brake work was accomplished. 1.13.2 Flight Information The aircraft departed St. Anthony with 800 pounds of fuel on board and an aircraft take-off weight of 7,240 pounds. The aircraft landed in Mary's Harbour with 685 pounds of fuel on board and an aircraft landing weight of 7,125 pounds. Although the aircraft centre of gravity (C of G) was within limits, the aircraft landing weight in Mary's Harbour exceeded the maximum approved landing weight by 125 pounds. The time interval between the completion of the Mary's Harbour arrival taxi phase and the commencement of the departure taxi phase was nine minutes. The PF completed the Pilot's Operating Manual checklist that included Brakes-Check Pressure prior to the Fox Harbour landing and reported that the brakes were normal.